The invention relates to a disposable multiwell filter apparatus for use in biological and biochemical assays that can be used and is compatible with existing equipment.
In pharmaceutical and biological research laboratories, plates with a multitude of wells have replaced traditional test tubes for assay and analysis. For many years, multi-well laboratory plates have been manufactured in configurations ranging from 1 well to 384 wells, and beyond. The wells of multi-well plates are typically used as reaction vessels in which various assays are performed. The types of analytical and diagnostic assays are numerous. The typical areas of use include cell culture, drug discovery research, immunology, and molecular biology, among others. Current industry standard multi-well plates are laid out with 96 wells in an 8xc3x9712 matrix (mutually perpendicular 8 and 12 well rows). In addition, the height, length and width of the 96-well plates are standardized. This standardization has resulted in the development of a large array of auxiliary equipment specifically developed for 96-well formats.
Many assays or tests require a mixture of particulate or cellular matter in a fluid medium. The mixture is then subjected to combination with reagents, separation steps and washing steps. The end product of such analysis is often a residue of solid matter which may be extracted for further analysis.
Separation of solids from fluid medium is often accomplished by filtration. The separation is accomplished in or on the filter material by passing the liquid through it. The liquid can be propelled through the membrane either by a pressure differential or by centrifugal force. Filter plates that conform to a 96 well standardized format are known as disclosed in U.S. Pat. Nos. 4,427,415 and 5,047,215. One significant problem that has been encountered with filter plates adapted for use with a 96 well plate is that cross contamination may occur between wells. When a unitary filter sheet is sandwiched between two pieces of plastic molded in a 96 well format, liquid from one well, upon wetting the filter material, may wick through the paper to neighboring wells thereby contaminating the sample contained within that well. One solution to this problem is offered in U.S. Pat. Nos. 4,948,442 and 5,047,215. In these patents, a 96 well filter plate is disclosed comprising a filter sheet placed between two plastic plates. One of the plates has a series of ridges that cut the filter sheet when the plates are ultrasonically welded together. By cutting the filter sheet around each well, the possibility of wicking between neighboring wells is effectively eliminated. A problem with this design is that it limits the product offering to membranes that can be cut by the process and to plate materials that can be ultrasonically welded. In fact, the potential for wicking and cross contamination still exists when the filter material is not completely severed in the welding process.
U.S. Pat. No. 4,427,415 discloses a filter plate of one piece construction having wells with drain holes in the bottom and capable of receiving filter discs into the wells. Wicking is obviously not a problem in this plate because individual filter discs are used as opposed to a unitary sheet of filter paper. The filter discs used in this plate are put into each well individually and are not secured to the bottom of the well. A danger exists with a filter disc that has not been secured down in that some liquid from the well could pass under the filter and thereby escape filtration, resulting in contamination of the filtrate.
Our invention solves several problems of prior art filter plate designs by providing a multiwell filter plate in which 1) filters are securely fastened to the plate without the use of glue or other potentially contaminant chemical adhesives, 2) an expansive variety of filter materials may be used, 3) a large number of thermoplastic components may be employed in its construction, and 4) no cross contamination through liquid wicking occurs between neighboring wells. The preferred embodiment of the present invention also offers a conical nozzle designed to cause exiting fluid to create droplets rather than lateral flow along the bottom of the plate. Further, a ring or skirt will preferably circumscribe the underside of each filter well. The skirt fits into a corresponding well of a receiver plate and is designed to prevent cross contamination that may otherwise occur by splashing of filtrate.
It is therefore an object of the present invention to provide a disposable filtration device for chemical and biological tests in which a large number of samples may be tested simultaneously. Further objects of the present invention are: to provide a filter plate that will be compatible with existing 96 well cluster plate formats as standardized by the industry; to provide a filter plate that can be handled by automated robotic assay equipment; to provide a filter plate having individual wells having a support grid on the bottom to help support the filter element, prevent tearing, and allow for an even distribution of filtered material on the filter; to provide a filter plate in which liquid from one well can not mix with liquid from a neighboring well (the filter plate of the present design prevents lateral flow or cross-talk of liquid through the membrane to other wells); to prevent cross contamination of filtrate after passing through the filter and passing to a receiver plate; to provide a filter plate of two part construction in which each individual well filter is securely pinned between opposing plates that are insert molded against each other; and to provide a unique method for the manufacture of filter plates.
Briefly, the present invention relates to an improved filter plate and its method of manufacture. The filter plate is a two part construction. It comprises a well plate preferably with 96 wells, each well being open on both ends, molded against a harvester plate insert preferably having 96 counter-bores, each containing a filter disc, whereby each counter-bore aligns with a corresponding and respective well from the well plate, and whereby the diameter of the counter-bore is greater than the diameter of the well, such that the well bonds with the outer rim of the counterbore thereby creating a lap joint. The lap joint also serves the purpose of fixing the filter disc securely to the insert without the need for glue or chemical adhesives. During the injection molding process, extremely high pressures in the mold ensure that the edges of the filter disc are pressed against the insert.
The assembled filter plate product has a plurality of interconnected wells of uniform diameter, each well being defined by a circular side wall, each of the side walls being interconnected to the side wall of at least two adjacent wells, each of the wells being open at one end. Further, the plate has a bottom wall at the bottom of each of the wells, which is connected to the side wall, each of the bottom walls having an opening therein. A conical drainage nozzle having an external surface and an internal passage communicating with the opening in the bottom wall, extends downwardly from the bottom wall from a point radially inward from the side wall. Finally, a filter disc is positioned on top of the bottom walls of the wells, the peripheries of each filter being sandwiched between a bottom portion of the side wall of each well and a top portion of the bottom wall of each well. The bottom walls have an opening therein, the opening preferably taking the form of a funnel shaped nozzle. A support grid preferably extends across the opening in order to provide support for the filter disc.
The method of manufacturing the plate comprises several steps, namely: forming an insert having a plurality of counter-bores; punching filter discs into the bottom surface of the counter-bore; and insert molding a well plate against the insert and filters such that wells from the well plate align with corresponding counter-bores from the insert thereby forming a lap joint that effectively secures the filter disc in place. The method can be extended for use in the manufacture of multiwell plates which do not have a filter, but require a well bottom of a different material than the side walls.